Such an electronic ballast which is known from the prior art is illustrated in FIG. 1. It comprises a control unit 10, which drives, in a known manner, a first switch S1 and a second switch S2 in a half-bridge arrangement. The switches S1, S2 are arranged between the so-called intermediate circuit voltage UZW and the ground potential, a half-bridge center point M being formed between the switches S1, S2, across which half-bridge center point M the voltage UM drops. A supply circuit, which comprises a capacitor C1 and three diodes D1, D2 and D3, the diode D3 being in the form of a zener diode, is used for producing a supply voltage for the control unit 10 from the voltage UM at the half-bridge center point M. A lamp La is connected to the half-bridge center point M via an inductance L1, the first connection 12 for the lamp La being connected to the ground potential via a coupling capacitor C4, and a second connection 14 for the lamp La being connected to the ground potential via a coupling capacitor C5. Moreover, the control unit 10 has an input 16 which is used for identifying capacitive switching of the switches S1, S2. Capacitive switching indicates undesirable switching of the switches S1 and S2, in which, at the same time, both the voltage and current occur at the respective switch S1, S2. The respective switch S1, S2 is therefore not free of power loss during the switching operation, which on the one hand results in its life being shortened and on the other hand results in an increase in the total power loss of the electronic ballast. So-called soft switching is desired in which a switch is only switched on once the polarity-reversal operation has been concluded. For this purpose, the voltage UM at the half-bridge center point M is applied to the input 16 of the control unit 10 via a capacitive voltage divider, which comprises the capacitors C2 and C3.
This solution known from the prior art entails a plurality of disadvantages: firstly: since the capacitor C2 is connected directly to the half-bridge center point M and is thus subjected to the intermediate circuit voltage UZW, which is generally 450 V, at specific times, this capacitor needs to be implemented in the form of a high-quality and thus cost-intensive capacitor, for example of the so-called MKP type, owing to the required reliability—in the same manner as the so-called snubber C C1. These capacitors need to have, in particular, high dielectric strength. Secondly: the signal evaluated in the prior art, namely the voltage UM at the half-bridge center point M, increases comparatively slowly. This is due to the fact that the capacitor C1 is initially, i.e. at the beginning of a polarity-reversal operation, not charged. Since in this case the capacitors C2 and C3 need to be charged parallel in time to the capacitor C1, this results in a delay which leads to a slow rise in the voltage UM at the half-bridge center point M. Since the maximum amplitude of the measurement signal is intended to be concluded in the control unit 10 on the basis of the amplitude of the measurement signal at the time at which one of the switches S1, S2 is switched on, in this case it is necessary to wait for a very long period of time in order to obtain a sufficiently high amplitude value as the basis for the estimation. A low amplitude value would lead to an imprecise estimation. Thirdly: owing to the measurement signal evaluated in the control unit 10, unnecessary and therefore undesirable disconnection operations may result in the case of sensitively set control units 10.